[Shipping labels] Add "id" parameter to purchase label endpoint

[irfano]

Description

This PR adds the required "id" parameter, which represents the shipment index, to the POST /wcshipping/v1/label/purchase/{order_id} endpoint. Related discussion: p1749620015063859-slack-C05VBLKHHV1

Steps to reproduce

1. Open an order in the app.
2. Tap the Create shipping label button.
3. Fill in address fields if missing.
4. Select a package and rate.
5. Tap the Purchase label button.
6. Restart the app, open the same order, and compare purchased shipment with the Create shipping labels page on the web.

The tests that have been performed

Steps above

Images/gif

• [x] I have considered if this change warrants release notes and have added them to RELEASE-NOTES.txt if necessary. Use the "[Internal]" label for non-user-facing changes.

[dangermattic]

	1 Warning
:warning:	This PR is assigned to the milestone 22.7. This milestone is due in less than 2 days. Please make sure to get it merged by then or assign it to a milestone with a later deadline.

Generated by :no_entry_sign: Danger

[wpmobilebot]

📲 You can test the changes from this Pull Request in WooCommerce-Wear Android by scanning the QR code below to install the corresponding build.

	App Name	WooCommerce-Wear Android
	Platform	⌚️ Wear OS
	Flavor	Jalapeno
	Build Type	Debug
	Commit	723d73a7658193670a160027e37ae59928e6bce9
	Direct Download	woocommerce-wear-prototype-build-pr14207-723d73a.apk

[wpmobilebot]

📲 You can test the changes from this Pull Request in WooCommerce Android by scanning the QR code below to install the corresponding build.

	App Name	WooCommerce Android
	Platform	📱 Mobile
	Flavor	Jalapeno
	Build Type	Debug
	Commit	723d73a7658193670a160027e37ae59928e6bce9
	Direct Download	woocommerce-prototype-build-pr14207-723d73a.apk